Sunday, January 19, 2014

Secret sign

I was walking down a steep mountain path in Sichuan with a local guide, paying rapt attention as he told me stories about the area. There were graves in the hillside, he said, and as a troublesome young man he once looked into them, and was terrified by the corpses. Another time he lost his favorite horse, who slipped on the path and fell to his death in a deep ravine. That complex in the valley was a prison, where he had spent some time.

Let's get together in the village later in the day, he said finally. But let's lose the Chinese guy. I don't trust him.

Indeed, I had been curious about the soft, overweight Chinese man in our party. He did not seem physically suited to a three-day horse ride, and he seemed to prefer reading stories on his cell phone to enjoying the dramatic scenery of the Sichuan mountains. Why was he there? My guide seemed to find it suspicious.

If anyone had been listening in, they would have been completely unaware of our conversation on the matter. This is because my guide was deaf, and we were communicating in Chinese Sign Language, of which I had managed to learn a fair amount over the prior three days.

In a previous post, I mentioned some qualities of a good secret language. Here, let me extol the virtues of sign language as an effective means of secret communication in the 21st century.

A secret language is, roughly speaking, a substitution cipher that operates on the level of morphology and grammar. Experience teaches us that unknown languages are difficult to decipher, so as long as the "key" remains a secret, the language remains relatively secure. The "key", in this case, is the combination of lexicon and grammar.

As a cryptographic system, secret languages are terrible. The key is difficult to transmit, and once broken, a new key must be laboriously created and transmitted. However, the great saving grace of secret languages in the 21st century is that encryption can take place entirely within the only device that remains free of malware: the human brain.

In order to remain secure, however, encryption must remain within the human brain. One of the significant weaknesses of secret languages in the era of the surveillance state is that users may be tempted to store or transmit the lexicon and grammar in an electronic form that may be intercepted and compromised. Another weakness is that keywords in the secret language may be distinctive enough that secret messages may be easily identified and used for traffic analysis.

A secret sign language is more secure on both of these counts. First, the key is actually difficult to store in writing, and is most naturally communicated person-to-person. Second, the easiest way to transmit a message over the internet is by video, which requires much more extensive and complex analysis even to pick out the existence of the secret communication.

Today's surveillance states have vast means at their disposal, and can easily out-spend and out-compute most of their adversaries. For the time being, however, there are a few faculties of the human mind that remain out of the reach of conventional computation. A secret sign language takes advantage of many of these capabilities, at a relatively cheap cost.

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